Objective:To explore the related mechanisms of biological root resorption in decidual teeth by studying the biological effect of simulated occlusal force on the periodontal ligament stem cells (PDLSC) at different stages of root absorption.Methods:According to the tooth type and root absorption degree, healthy retained deciduous incisors and healthy first premolars that needed to be removed for orthodontic treatment were collected and divided into three groups with six teeth in each group: the deciduous unabsorbed group (UN group), the absorbed group (R group) and the permanent teeth group (P group). PDLSC was isolated from periodontal ligament and cultured. PDLSC of three groups were loaded with dynamic pressure of 0-45, 0-90, 0-135, 0-180, 0-225 and 0-270 kPa, respectively. The proliferation ability was detected by cell counting kit-8 (CCK-8) technique on day 1 to day 7, respectively. The apoptosis levels of PDLSC after loading with dynamic pressure of 0-45, 0-90, 0-135, 0-180 and 0-225 kPa were observed by the flow cytometry. The changes of microfilaments were observed by fibrous actin (F-actin) staining after the cytokeleton was subjected to dynamic pressure of 0-90 kPa.Results:PDLSC of three groups exhibited various proliferation abilities to dynamic pressure. The A values in the UN group and R group were significantly higher than those in the P group and the difference was statistically significant ( P<0.05). However, there was no significant difference between the UN group and the R group ( P>0.05). The A values of PDLSC in UN group and R group under dynamic pressures of 0-45, 0-90, 0-135 and 0-180 kPa had no statistical significance compared with the control group unloading dynamic pressure ( P>0.05). However, under 0-225 and 0-270 kPa dynamic pressures, the A values at the day 3 to day 7 were statistically significant ( P<0.05). The A values of PDLSC in P group under 0-45, 0-90, 0-135, 0-180 and 0-225 kPa dynamic pressures for 1 to 7 days were no statistically significant difference compared with the control group ( P>0.05). The A value in P group under the 0-270 kPa was statistically significant only on day 3 (1.386±0.131) and day 5 to day 7 (1.728±0.226, 2.029±0.168 and 2.263±0.210, respectively)( P<0.05). The result of apoptosis showed that the A values of PDLSC in UN group, R group and P group were significantly increased under 0-90, 0-135, 0-180 kPa and above dynamic pressures, respectively ( P<0.05) compared with the control group unloading dynamic pressure. Under 0-90 kPa dynamic pressure, F-actin fluorescence staining samples in three groups all showed green filaments which were arranged along the long axis of the cells in the R group and the P group, while some fibers in the UN group were closely arranged and promoted stress fiber assembly. Conclusions:The biological characteristics of PDLSC at different root absorption stages were changed when they were stimulated by mechanical stress, and PDLSC of the deciduous teeth at the root unabsorption stage were more sensitive to mechanical stress stimulation.